During Release 12, the LTE standard has been extended with support of device to device (D2D) (specified as “sidelink”) features targeting both commercial and Public Safety applications. Some applications enabled by Rel-12 LTE are device discovery, where devices are able to sense the proximity of another device and associated application by broadcasting and detecting discovery messages that carry device and application identities. Another application consists of direct communication based on physical channels terminated directly between devices.
D2D communications may be extended to support Vehicle-to-X (V2X) communications, which includes any combination of direct communication between vehicles, pedestrian carried devices, and infrastructure mounted devices. V2x communication may take advantage of available network (NW) infrastructure, although at least basic V2x connectivity can be possible in case of lack of available network infrastructure. Providing an LTE-based V2x interface may be economically advantageous because of the LTE economies of scale and it may enable tighter integration between communications with the NW infrastructure (vehicle-to-infrastructure (V2I)), vehicle-to-pedestrian (V2P), and vehicle-to-vehicle (V2V) communications, as compared to using a dedicated V2x technology.
V2x communications may carry both non-safety and safety information, where each of the applications and services may be associated with specific requirements sets, e.g., in terms of latency, reliability, capacity, etc.
The European Telecommunications Standards Institute (ETSI) has defined two types of messages for road safety: Co-operative Awareness Message (CAM) and Decentralized Environmental Notification Message (DENM).
A CAM message is intended to enable vehicles, including emergency vehicles, to notify their presence and other relevant parameters in a broadcast fashion. Such messages target other vehicles, pedestrians, and infrastructure, and are handled by their applications. The CAM message also serves as active assistance to safety driving for normal traffic. Devices check availability of a CAM message every 100 ms, yielding a maximum detection latency requirement is not more than 100 ms for most CAM messages. However, the latency requirement for Pre-crash sensing warning is not more than 50 ms.
A DENM message is event-triggered, such as by braking, and the availability of a DENM message is also checked for every 100 ms, and the requirement of maximum latency is not more than 100 ms.
The package size of CAM and DENM message can vary from more than 100 to more than 800 bytes, although the typical size is around 300 bytes depending on the specific V2X use case, message type (e.g. DENM can be larger than CAM), and depending on the security format included in the packet (e.g., full certificate or certificate digest). The message is supposed to be detected by all vehicles in proximity.
The Society of the Automotive Engineers (SAE) has defined a Basic Safety Message (BSM) for DSRC with various defined messages sizes. Based on the importance and urgency of the messages, the BSMs are further classified into different priorities.
Sensing-Based Resource Allocation with Booking
In V2x communications, two major types of traffic are distinguished: recurrent traffic and event-triggered traffic. Various embodiments disclosed herein are mostly related to recurrent traffic, where the transmitted packets arrive regularly (e.g., they may be strictly periodic or have some deviation from an average periodicity).
One efficient way to schedule recurrent-traffic V2x transmissions is to use radio resource booking. In resource allocation using resource booking a user equipment (UE) can book radio resources in advance for transmitting the next packet (including all the retransmissions). The minimum time span of a booking is usually taken to be the minimum time between two consecutive packets (e.g., the minimum message periodicity). Similarly, the maximum time span of a booking is usually taken to be the maximum time between two consecutive packets (e.g., the maximum message periodicity). For example, in V2X the time interval between the generation of two consecutive CAM messages may not be shorter than 100 ms (in the absence of congestion control) and may not exceed 1 s. Thus, it is reasonable to allow bookings for 100 ms, 200 ms, . . . , or 1 s, as it is currently being considered by 3GPP. Usually, the UE signals the booking information to other UEs. This allows a receiving UE to predict the future utilization of the radio resources by reading received booking messages and schedule its current transmission to avoid using the same resources. To do so, a UE needs to sense the channel for some time duration preceding the (re)selection trigger to gather booking messages. In addition, it may also be possible to transmit unhooking messages that release previously booked resources. For accurate prediction, the sensing time should be long enough to detect booking and/or unbooking messages from other relevant UEs.
FIG. 1 illustrates an example of the sensing-based resource selection mechanism with booking. Let T be the minimum time between two recurrent transmissions by a UE, which we refer to as “basic period”. That is, a UE with recurrent traffic transmits, at most, one packet every T seconds (a transmission may consist of several retransmissions, although this is not illustrated in FIG. 1 for simplicity). In FIG. 1, UE 1 transmits a packet at time to and meanwhile books—e.g., transmits a booking message to other UEs indicating—its intention to transmit the next packet at ta+4T. Similarly, UE 2 transmits a packet at time tb and meanwhile books—e.g., transmits a booking message to other UEs indicating—its intention to transmit the next packet at tb+2T. At time tc, UE 3 wants to select or reselect a radio resource for its transmission within the time window [tc,tc+T]. UE 3 has been monitoring the channel during a time window of size 4T. UE 3 uses its channel observations in this window to predict the future utilization of the radio resources and accordingly select a radio resource for its transmission (e.g., a resource that is not indicated by the above bookings to avoid potential collision).
It is clear that to achieve good performance the sensing window must be long enough to include as many bookings as possible or necessary. Commonly, the size of the sensing window is sufficiently large to roughly cover the longest possible booking (in terms of basic periods). In the example in FIG. 1, the sensing window is chosen to consist of four basic periods (“T” in FIG. 1). In the remainder of this disclosure, the expressions “sensing over the entire window” and “sensing over the whole window” refer to performing the sensing operation using the largest possible window size (i.e., the largest window size that the system allows for).
It is noted that in this example and in the rest of this disclosure, UEs may or may not operate using a common division of the time in terms of basic periods. That is, time may be divided into “basic periods” in the same way for all UEs or, alternatively, different UEs may have different divisions of time into “basic periods”.
Sidelink Operations
Sidelink transmissions (also known as D2D or ProSe) over the so-called PC5 interface in cellular spectrum have been standardized in 3GPP since Rel-12. In 3GPP Rel-12 two different operative modes (sometimes referred to as transmission modes) have been specified in 3GPP. In one mode (mode-1), a UE in RRC_CONNECTED mode requests D2D resources and the eNB grants them via PDCCH (DCIS) or via dedicated signalling. In another mode (mode-2), a UE autonomously selects resources for transmission from a pool of available resources that the eNB provides in broadcast via SIB signalling for transmissions on carriers other than the PCell or via dedicated signaling for transmission on the PCell. Therefore, unlike the first operation mode, the second operation mode can be performed also by UEs in RRC_IDLE.
In 3GPP Rel.14, the usage of sidelink is extended to the V2x domain. The design of the sidelink physical layer in Rel.12 has been dictated by the assumptions of few amount of UEs competing for the same physical resources in the spectrum, to carry voice packet for MCPTT traffic, and low-mobility. On the other hand, in V2x the sidelink should be able to cope with higher load scenario (i.e. hundreds of cars could potentially contend physical resources), to carry time/event triggered V2x messages (CAM, DNEM), and high mobility. For such reasons, 3GPP has discussed possible enhancements to the sidelink physical layer.
An enhancement of the physical layer has been proposed for the UE to perform resource selection on mode-2 to enhance the UE autonomously selecting resources for transmission. Before 3GPP Rel-14, the UE performs resource selection randomly, i.e. the eNB provides a pool of time/frequency resources and the UE randomly pick a subset of resources from such pool to perform sidelink transmission. With the introduction of V2V in 3GPP Rel.14, the UE performs sensing before transmitting. Unlike random selection, with sensing the UE monitors the sidelink resources for a certain amount of time, for example, 1 second, before performing transmission. In this way, the risk of transmission collisions with other UEs in the surrounding area may be significantly reduced compared with random selection since the UE selects the resources that according to the sensing procedure are supposed to be less interfered.
The sensing procedure implies that a UE can book transmitting resources for future transmissions. The booked resources are announced to surrounding UEs in the sidelink control channel Upon receiving such sidelink control channel information surrounding UEs may estimate the interference for a future resource allocation, and on this basis, may determine whether to transmit or not in such corresponding resource allocation. Multiple booking processes may be performed by the UE.
In 3GPP Rel.14, two new modes (sometimes referred to as transmission modes) were introduced for LTE sidelink: mode-3 and mode-4. Mode-3 is an extended version of mode-1 targeting V2x communications. In mode-3, resource allocation is performed by the eNB. Mode-4 is an autonomous mode in which the UE selects its own resources for transmission. Selection in mode-4 is based on the sensing and makes use of resource reservation, as described above. Although mode-4 UEs select resources autonomously, the eNB may retain some degree of control over them. For example, it may configure a specific set of resources to be used by mode-4 UEs; or it may prevent mode-4 UEs from accessing certain resource.